This invention relates to a thermophotovoltaic (TPV) apparatus for generating electricity using an emitter with multiple spectral bands matched to the absorption bands of a multilayer photovoltaic device.
Electric power can be generated by heating a radiation emitter to a sufficient temperature that it will emit infrared or visible radiation. Such radiation is absorbed by a photovoltaic device such as a silicon cell to produce an output voltage and current. The emitter may be a blackbody or "grey body" with typical broad band thermal emission. Alternatively, it may be a thermally stimulated quantum emitter which produces radiation in a relatively narrow spectral band when heated above a threshold temperature. A variety of photovoltaic devices are commerically available for absorbing such radiation.
U.S. Pat. No. 3,188,836 by Kniebes describes utilization of emissive radiation from a glowing mantle in a gas lamp to generate sufficient power to control a valve. U.S. Pat. No. 3,331,701 by Werth provides a description of a thermophotovoltaic power producing device. R. M. Swanson in "Silicon Photovoltaic Cell in TPV Conversion," ER 12712, Project 790-2, Stanford Univ., (December 1979), who pioneered the fundamentals of blackbody thermal voltaic devices, describes an efficient solar cell. This cell work was initiated to optimize the performance of silicon cells when used in conjunction with a blackbody emitter. Swanson has also reported that these cells produce electric power with an efficiency of 26% using a tungsten filament heated to about 2200.degree. K. as the heat source. Proceedings of IEEE, 67 (1979) 446; ER-1277, Project 790-2, Stanford Univ., (December 1979).
The efficiency of a narrow band emitter operating at the same power level should be much greater than where power is generated from radiation from a blackbody emitter. U.S. patent application Ser. No. 659,074 filed Oct. 5, 1984, and Ser. No. 701,369, filed Feb. 13, 1985 both assigned to the same assignee of this application, and European Patent Publication No. 84 306033.6 by Nelson, and European Patent Publication No. 83 108018.9 by Diederich disclose advantages of using rare earth metal oxide narrow band emitters matched to the absorption characteristics of photovoltaic devices to produce efficient energy conversion for use in various gas appliance control and power production applications. Application Ser. No. 659,074 was a National Application corresponding to International Application PCT/US84/01038 filed July 3, 1984, which was a continuation-in-part claiming priority of U.S. patent application Ser. No. 517,699, filed July 25, 1983. The aforementioned patent applications are now abandoned.
British Pat. No. 124 by Carl Auer von Welsbach was the origin of the first successful gas light mantle almost 100 years ago. That structure comprises a thermally stimulated quantum emitter comprising ceria. Such a mantle is designed to emit a broad band spectrum of white light rather than a narrow band. In "High Temperature Spectral Emitters of Oxides of Erbium, Samarium, Neodymium, & Ytterbium", Applied Spectroscopy, 26 (1972) 60-65, Guido Guazzoni suggested the use of narrow band emitters for electric power production. His data suggested that the spectral emittance of ytterbia (Yb.sub.2 O.sub.3) is particularly well suited for use with silicon photovoltaic devices in a power production system.
The next major steps in the development of thermal voltaic power technology involved improvements in materials science. In European Patent Publication 84 306033.6, R. E. Nelson describes a small strong mantle capable of withstanding 1000 g. This is almost 100 times stronger than the Welsbach type mantle. In U.S. patent application Ser. No. 864,088, filed May 16, 1986, Goldstein shows another approach for strong emissive devices for implementing large scale cogeneration appliances.
For the past several years there has been appreciable research on blackbody thermophotovoltaic devices reported in R. N. Braewell and R. M. Swanson, "Silicon Photovoltaic Cells in TPV Conversion," EPRI ER-633, (February 1978); J. C. Bass, N. B. Elsner, R. J. Meyer, P. H. Miller, Jr., and M. T. Sinmad, "Nuclear-Thermophotovoltaic Energy Conversion," NASA CR-167988 (GA-A16653) G. A. Technologies, Inc., (December 1983); L. D. Woolf, J. C. Bass, and N. B. Elsner, "Variable Band Gap Materials for Thermophotovoltaic Generators," GA-A18140, GA Technologies, Inc., (December 1983); and papers mentioned above. In these documents, Swanson indicates that efficiencies of at least 50% may be possible and he has measured conversion efficiencies of about 30% with a relatively crude experimental setup using a blackbody emitter. Fahrenbruch states that the photovoltaic conversion efficiency when using an emitter which emits narrow band radiation, may be much greater than that obtained using blackbody radiation.
The reason for this improvement in conversion efficiency is that the energy required to promote an electron from the conduction band to the valence band is equivalent to a specific quantity of energy or wavelength, the band gap energy. For each photon absorbed by the photovoltaic device, one electron is promoted into the conduction band. If the photons absorbed have energy in excess of the band gap energy, the excess energy is converted into heat or phonons and this decreases the conversion efficiency. It is, therefore, desirable to absorb radiation with minimal deviation from the band gap energy.
Ytterbia is a narrow band emitter which emits photons over a narrow range of energies with a band width of 50 to 100 nanometers centered at about 950 to 1000 mm. Use of this emitter material produces a substantial improvement in the thermophotovoltaic energy conversion efficiency when compared to the use of blackbody emitters and may lead to the development of many practical devices for generation of electric power. As shown by Nelson in U.S. Pat. No. 4,584,426, the emissive output over the range of from 400 to 2500 nanometers should have 50% of the radiant energy within a single band.
It is clearly of significance in power generating system to enhance the thermal to light conversion efficiency. Thus, it is desirable to enhance the proportion of the thermal energy that goes into heating the emitter which is finally absorbed by the photovoltaic devices and efficiently converted to electric power.